Modern integrated circuits are typically made of multiple layers of materials, such as metals, semiconductors, and insulators, fabricated on semiconductor substrates. To characterize the fabrication process or investigate process problems in semiconductor manufacturing, process engineers often use a focused ion beam to cut a cross section of the integrated circuit and then use a scanning electron microscope to view the edges of the layer in the cross section. For example, process engineers may cut cross sections to observe integrated circuit features, such as poly-silicon gates, amorphous silicon, and various dielectrics and barrier materials and to measure the layer thickness and uniformity.
A scanning electron microscope forms an image by collecting secondary electrons that are emitted from a surface as the beam is scanned across it, with the brightness at each point of the image being proportion to the number of secondary electrons (or another electron signal) collected from the corresponding point on the surface. The number of secondary electrons emitted at each point depends on the type of material and on the topography. Because many different types of materials emit different numbers of secondary electrons per incident electron, it is easy to observe a boundary, for example, between a metal layer and an oxide layer. Some similar materials, such as oxides and nitrides, emit about the same number of secondary electrons for each primary electron, and so the boundary between those materials is often not apparent in the electron beam image.
One method of making the interface visible is to selectively etch the area of the interface. If one material etches more quickly than the other material, there will be a change in topography at the interface, which will be visible in the image. Processing a work piece to make a feature more visible is referred to as “decoration.” One decoration process from the assignee of the present invention is referred to as the Delineation Etch™ process, which comprises chemically assisted focused ion beam etching using a fluorinated hydrocarbon vapor of 2,2,2-trifluoroacetamide. When making an interface visible, it is desirable to change the structure as little as possible so that the process engineer obtains an accurate picture of the work piece. Very low etch rates are therefore desirable to allow the process engineer precise control over the decoration process.
As feature sizes in integrated circuits decrease, the inherent damage to the sample caused by ion sputtering introduces measurement error that may not be tolerable. When the decoration is performed using an electron beam instead of an ion beam, physical sputter damage is eliminated. Because of the negligible momentum transfer in electron-beam-induced reactions, an electron beam typically etches only in the presence of an etchant precursor gas.
Background gases in a vacuum chamber often contain carbon and cause carbon deposits during electron beam processing. The electron beam-induced etching must outcompete electron beam-induced deposition from background gases or no etching will occur. Due to its aggressive fluorinating properties, xenon difluoride (XeF2) is often used with an electron beam to etch silicon-containing materials, as well as refractory metals. Unfortunately, XeF2 spontaneously etches silicon at a higher rate than the electron beam-induced etching of silicon dioxide (SiO2). Thus, when attempting to etch SiO2, in the presence of exposed silicon, the gas produces more unintentional damage than desired etching. Moreover, the decoration produced by electron beam etching between layers of different types of dielectrics is inadequate for most applications. For example, electron beam etching with XeF2 etches nitride layers and oxide layers at a similar rate and so an observer cannot readily observe the boundary between those layers. In addition, the overall rate may be too high to have adequate control over the etch depth, even if beam-induced selectivity is observed. To further illustrate this point, consider a very fine nitride feature buried in oxide as the sample to be decorated. In this case, a very soft nitride etch would be preferable (due to small feature size), while more rapidly removing the surrounding oxide. Conversely, the situation could be reversed, where the higher nitride etch rate is preferable. In these types of cases, it is highly desirable to have a specific chemistry capable of providing selectivity for the desired material Another type of use case is where there may be exposed silicon (poly-, amorphous, or single-crystal) that should be preserved in the region of interest to be decorated. In this case, silicon etching is almost solely spontaneous, therefore this case requires more of a spontaneous etch suppression as opposed to a modified beam-induced etch rate. Again, a chemistry capable of all of the above is high desirable.
Another process used in semiconductor manufacturing is reactive ion etching. In reactive ion etching, a wafer is placed in a plasma chamber, where the plasma is comprised of ions, radicals, and neutrals aimed at increasing the gas reactivity. The reactive ions are accelerated toward the wafer surface and react both chemically and by a momentum transfer process. Reactive ion etching occurs on the whole wafer. The reactive ions are not focused, but are created throughout the plasma and accelerated toward the sample. The reaction can be controlled by controlling the gas species, some of which deposit onto the substrate and some of which etch the substrate.
What is needed is a way to selectively induce etching in a small region of a substrate by using a focused beam, so that the etching is controlled by the operation of the beam and by the types of materials on the substrate and is not significantly altered by physical processes such as ion-induced sputtering.